The present invention relates to the field of composite materials comprising a polymer matrix reinforced by a fiber structure, and more particularly to the use of such materials in the fabrication of aviation parts or of turbine engines.
In the field of aviation, it is desired to reduce the weight of engine components while maintaining their mechanical properties at a high level. More particularly, in an aeroengine, the fan casing, which defines the outline of the air inlet passage into the engine and inside which there is received the rotor supporting the blades of the fan, is nowadays made out of composite material. In the form of a body of revolution, it comprises a shroud provided at its upstream and downstream ends with outwardly-directed flanges for fastening to other structural portions of the engine, such as the air inlet profile upstream, and the intermediate casing downstream. The casing also supports various components, and it must be capable of retaining the debris that results from a fan blade breaking or from items being ingested at the inlet of the engine.
Fabricating a fan casing out of composite material begins by putting a winding of fiber reinforcement into place on a mandrel having a profile that matches the profile of the casing that is to be made. By way of example, the fiber reinforcement may be made by three-dimensional or multilayer weaving, as described in patent U.S. Pat. No. 8,322,971. This fiber reinforcement constitutes a tubular fiber preform that is made integrally with margins corresponding to the flanges of the casing. Fabrication continues with the fiber preform being densified by a polymer matrix, where this consists in impregnating the preform with a resin and polymerizing the resin in order to obtain the final part.
The invention relates more particularly to fabrication in which the fiber preform is impregnated by the resin transfer molding (RTM) injection molding method. In this method, the fiber preform is enclosed by compacting it in a rigid mold of unchanging shape that has an inner half forming a support for the fiber preform and an outer half that is placed on the fiber preform and that is of shape that corresponds to the shape of the casing that is to be obtained, after which resin is injected under pressure and at controlled temperature into the inside of the mold after the walls of the two mold halves have been brought together, and possibly after the mold has been evacuated. Once the resin has been injected, it is polymerized by heating the mold, and after injection and polymerization, the final part is unmolded and then trimmed in order to avoid excess resin, and chamfers are machined in order to obtain the desired casing.
In order to facilitate such unmolding, the injection tooling needs to be completely leaktight, and in general it is constituted by a mold comprising: two adjacent inside drums reproducing the passage for air; upstream and downstream cheek plates for forming the two rims of the casings; and an outer half made up from a plurality of outer sectors. These sectors and the upstream and downstream cheek plates are removed outwards. The two inner drums are uncoupled and then removed from each side of the casing relying on the natural drafts of the air passage.
Unfortunately, in certain casing configurations, the passage for air narrows at the upstream end of the casing, with the passage varying, for example, between an upstream first diameter to a second diameter that is larger in a central zone, and once more to a smaller third diameter further downstream, which means that the final part cannot be unmolded because of the impossibility of withdrawing the upstream drum. The difference between the small and the large diameters may for example lie in the range 40 millimeters (mm) to 80 mm for a casing having a mean diameter that may be about 1500 mm to 3500 mm.